Madison Sinclair is the "resident bitch" of Neptune High. She epitomizes the 09er clique: She is shallow, spoiled, and dated Dick Casablancas, one of Neptune High's most arrogant and self-centered young men. She also single-handedly rigged a school election in order to get Duncan Kane elected to student body president. Veronica exposed the plot which resulted in Madison losing her place on the student council and the special privileges granted by the position.
Madison likes to spit in the drinks of people she doesn't like, which she calls "A Trip to the Dentist." When she spit into a cup of spiked drink meant for her and gave it to Veronica, she set off a chain of events that led to Veronica being raped. When Dick kissed Veronica in her drugged state, she defaced her car with shoe polish, writing the words "SLUT!" and "Abel, it should have been HER!".
Despite not being with Dick anymore, she still had feelings for him at the beginning of Season Two. While at a slumber party hosted by Gia Goodman, she learned that Gia was now dating Dick and was visibly upset. However, by the end of the season, she appears to be over him as she is involved secretly with Sheriff Don Lamb (revealed in 2x16 "The Rapes of Graff"). She publicly humiliated Jackie Cook at the winter carnival after Jackie ignored her warning to stay home in "Ain't No Magic Mountain High Enough". She taunted a pep squad member after her sexuality was revealed to the school in "Versatile Toppings".
Veronica had identified Madison to her father when Keith Mars discovered Lamb was dating a mysterious blonde woman. She then ruthlessly mocked Madison about her relationship with Lamb in front of their classmates at the alterna-prom through innuendo that, while her classmates didn't understand, had a meaning that was quite apparent to Madison, who was visibly shaken as a result.
Madison, having gone off to college at USC, is absent from much of Veronica's life at Hearst, until she had several run-ins with her in Neptune in the episode "Poughkeepsie, Tramps and Thieves". Veronica first sees Madison when she arrived unexpectedly at Neptune Grand Suite shared by Logan and Dick; thinking that Madison was simply looking up her ex, Veronica sent her on her way.
Later, however, she ran into her again at a lingerie shop where Madison maliciously let slip that she had spent the night with Logan while in Aspen over Christmas break, which is when Veronica and Logan were split up.
In the following episode, Veronica, consumed by images of Madison's tryst with Logan, follows her and eventually plots with Weevil to have Madison's new car crushed and cubed. Veronica has a change of heart, however, and asks him to return it unscathed (save for a can of tuna in her air conditioning). She becomes a Pole Cardio Instructor to stars years after.
Veronica encounters Madison in the movie at the Neptune High Reunion. Madison hasn't changed in the slightest, showing disdain to Veronica's presence and even showing her sex tape on the screen, starting a fight between Logan and some others. Veronica ultimately turns on the sprinklers, ending the reunion abruptly. Madison attempts to taunt Veronica once more, daring her to tase her with her Stun Gun, only to get punched in the face, finally getting her own trip to the dentist.
Madison returns in the second novel Mr. Kiss and Tell, where she nearly exposes Veronica during an investigation that leads her to the office of Madison's father Charles. Luckily, Veronica gets the information she needs and makes a quick exit just as Madison confronts her father about letting Veronica into the building.
Madison was switched at birth with Mac, one of Veronica's friends. As such, the self-absorbed Madison is often at odds with her more cerebral parents and younger sister, though her parents have gone out of their way to spoil Madison rotten. However, in spite of all of this, Madison's "mother" does harbor some resentment towards her daughter and longing to know her true biological daughter Mac. Ironically though, Mac's parents have fully accepted Mac as their daughter and have shown no signs of wanting anything to do with Madison.
- Veronica Mars